DC motor with increased rotor resistance

ABSTRACT

A brushless DC motor in which increased rotor resistance is used to facilitate very frequent reversals. The rotor endcap is thinned down to the point where the resistance seen by the path of the current loop through one of said endcaps, is at least one-half as much as the resistance seen by the portion of said current loop which flows along the length of one of said rotor bars.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to structures and control systems foroperation of small motors, and particularly structures and controlsystems for operation of vehicular windshield wipers.

Windshield Wiper System without Reversing Linkage

The present disclosure describes a wiper system for the transportationindustry that does not have cams, eccentrics or other complicatedmechanical means to reverse the direction of the wipers.

This is based on a low cost variable reluctance or similar typebrushless motor technology. (A "variable reluctance motor" is a motorwith rotor lobes of ferromagnetic material, usually soft iron). Byselectively exciting a stator field in the correct position, a rotorlobe can be pulled toward regions of increasing magnetic fieldintensity. Motion of a conductive rotor through the stator field will ofcourse induce secondary current in the rotor, so that this type of DCmotor has very significant similarities to an (AC) induction motor. Thispolyphase motor will typically be operated as though not limited tothree phases. For low driver cost, the stator would be wound such thatthree single ended drivers would be used. The rotor can be a normalrotor, but a modified one (according to other innovative teachings wouldbe much better.

Dedicated Microprocessor Control of Wiper Operation

According to certain innovative teachings disclosed herein, amicroprocessor is used to directly control operation of the wiper motorswhich does not include any reversing linkage.

By direct control of switching transistors to drive a reversiblebrushless motor, the microprocessor has full control of wiper motoroperation: torque is controlled by current (i.e. by the duty cycle ofthe power transistors), speed of the motor by frequency, and position byrotor-field relationships.

This can be implemented with a microcontroller, a few MOSFETs, and a fewresistors. Of course, other microcontrollers or microprocessors can alsobe used, or a fully integrated solution can also be used.

Control of Multiple Wiper Motors

Note that the programmable control capability provided by the presentinvention can also be used to control multiple wiper motors with asingle dedicated microcontroller. This eliminates linkage losses, andalso (in some embodiments) provides an added safety feature: the wipersand motors can be positioned to operate redundantly, so that the otherwipers will continue operating if one first wiper motor fails.

A further advantage of the disclosed innovations is that more than twowipers can easily be designed in. Traditionally, three-wiper systemshave been fairly uncommon in passenger cars, but design of such systemsis now greatly simplified by the disclosed innovations. Blade positionscan easily be coordinated, and multiple blade synchronization patternscan automatically selected depending on road conditions.

Shared Dedicated Controller for Wiper Motor and Washer Pump

The controller is also capable of driving another brushless or brushtype motor for the washer solution. By directly driving the phases ofthe washer solution motor, a lower cost brushless motor can be used.This will eliminate all the problems of brushes, such as wear-out,arcing, etc. Variable volumetric delivery is easily implemented withoutany additional hardware.

Using this smart control architecture, many additional optimizationalgorithms can be added. For example, the wiper speed and the deliveryof washing fluid can be made directly dependent on the vehicle speed.

Communication capabilities can easily be implemented, and (though notlimited to such) can be implemented with any of the normal automotivebus configurations, such as PWM, VPWM, JT1850, SCP, DLC, or PCMI.

Variable Downforce

In a further class of embodiments, a solenoid or other means is used tohelp the wiper blades maintain contact with the windscreen.Communications with other modules in the vehicle would allow properforce to be exerted by the blades, compensating for speeds. This can becontrolled by analog or digital means, such as PWM.

FIG. 5 shows the linkage used, in the presently preferred embodiment, toprovide variable downforce on the wiper blades.

An advantage of this--as opposed to using maximum downforce all thetime--is a reduction in the chances of scratching the windshield withabrasive bits of road dirt. A further advantage is the ability to use amaximum downforce which could stall or burn out the wiper motor ifapplied under conditions where there was no lift on the wiper blades. Afurther advantage is the ability to adapt wiper operation for winterconditions when ice or snow must be cleared from a windshield.

DC Motor with Increased Rotor Resistance

The present application discloses a modified low-speed DC motor whichparticularly adapted for frequent reversal. The modification is theopposite of what most motor manufacturers seek to do: the resistance ofthe rotor is intentionally increased. This can be implemented bychanging the material and/or size of the bars. It can also beaccomplished by increasing the resistance of the bar-shorting mechanismtypically connecting the bars at the ends of the rotor. This would beeasy to implement by decreasing the size and/or changing the materialused to short the bars.

This will cause the motor to lose its high speed characteristics andbecome more lossy and would result in a higher operating temperature.These effects would be more than compensated by the gains acquired atthe low RPM portion of the spectrum, particularly with the constantreversal of motor rotation as required in this type of application.

By using both the battery voltage sense and current sensing, a completeprofile can be generated. Conditions such as stalled motor, overloads,etc., can be detected and compensated using well-known algorithms. Thisgives the controller the capability to drive the motor at maximum speedfor a period of time without burning it up. If the load increases ordecreases, the drive can compensate for this as well as varying batteryvoltages.

Infinitely adjustable increment wiper action is implementable at noadditional hardware cost and a minimal impact on the software. Thesensing for the settings can be digital or analog.

It should be noted that a variable-reluctance motor has zero holdingtorque, so a magnet (or possibly a clutch) is preferably used to holdthe wiper in its "off" position when the motor is not active.

An advantage of this design is that no other modifications have to bemade to the motor besides the increased rotor resistance. Thus, minimalincremental change is needed to established motor designs andmanufacturing flows.

General background on electric motors is found in Anderson et al.,ELECTRIC MOTORS (5.ed. 1991); F. Spreadbury, FRACTIONAL H.-P. ELECTRICMOTORS: THEIR PRINCIPLES, CHARACTERISTICS AND DESIGN (1951); both ofwhich are hereby incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIG. 1A shows the shorting bar arrangement of a comparable conventionalmotor, and FIG. 1B schematically shows the modified shorting bars,according to one class of innovative embodiments, which provide apolyphase motor with improved capability for rapid reversals, at thecost of reduced efficiency and high-speed performance.

FIG. 2 shows a system block diagram of the presently preferredembodiment.

FIGS. 3A and 3B show plan and isometric views of a conventional motorrotor, and FIGS. 3C and 3D show corresponding plan and isometric viewsof an innovative motor rotor modified according to the innovationsdisclosed herein.

FIG. 4 shows the simple mechanical power train permitted by a sampleembodiment of the linkage-less wiper motor train.

FIG. 5 shows the electromechanical linkage used to provide variableholddown force to the wiper blades.

FIG. 6 shows a system block diagram of a system embodiment where asingle microcontroller directly controls multiple wiper motors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiment. However, it should be understood that this class ofembodiments provides only a few examples of the many advantageous usesof the innovative teachings herein. In general, statements made in thespecification of the present application do not necessarily delimit anyof the various claimed inventions. Moreover, some statements may applyto some inventive features but not to others.

Mechanical Configuration

FIG. 4 shows the simple mechanical power train permitted by a sampleembodiment of the linkage-less wiper motor train. A brushless motor 400directly drives a wiper shaft 420 (which preferably is simply anextension of the motor shaft). A conventional wiper arm 430 and wiperblade 440 are mounted to the shaft 420 by a conventional directcoupling.

FIG. 5 shows the electromechanical linkage used to provide variableholddown force to the wiper blades. A solenoid 510 (driven by anotherpower transistor, not shown, controlled by the microcontroller 100) hasa shaft which is coaxial with the shaft 420' of motor 400', and which isconnected to a tension cable 520 which passes through the axis of thewiper shaft 420'. The wiper arm 430' is directly driven (in rotation) bythe wiper shaft 420', but also has some freedom of motion to pivottoward or away from the windshield. The tension cable 520 combines withfixed mechanical biasing elements (schematically represented by a spring522) to exert downforce on the wiper blade 440.

The solenoid 510 can have fixed coils aligned to its rotating shaft, orthe whole solenoid can be mounted to the wiper shaft 420' to rotate withit. This could be accomplished with flexible wiring to the solenoid, ormore preferably (in the presently preferred embodiment) an inductivecoupling coil 550. (Current in the fixed coupling coil 550, which iscoaxial with the solenoid 510, will induce secondary current in thewinding of solenoid 510.)

Of course, other mechanical arrangements can be used to provide thevariable downforce; the arrangement shown is merely a simple example ofthe numerous configurations which can be used. (For example, a motorcould be used instead to provide a variable preload to a spring.)

The variable downforce connection can be used to only to increasedownforce for difficult conditions, but also to decrease downforce tofacilitate wiper starting in winter conditions.

A further mode of operation, which may be advantageous in winterconditions, is to start with wiper motion within a small arc at thewarmest spot in the windshield, and then expand the arc of motiongradually as the mechanical resistance (measured by motor current draw)indicates that the cleared area is expanding.

Electrical Configuration

FIG. 2 shows a system block diagram of the presently preferredembodiment. Microcontroller 100 may be, for example, an ST9030microcontroller from SGS-Thomson Microelectronics Inc. (Of course, othermicrocontrollers or microprocessors can also be used, or a fullyintegrated smart-power solution can also be used.)

Microcontroller 100 includes at least three digital outputs D1-D3, eachof which drives the gate of one of the three power MOS transistorsT1-T3, each of which pulls current through one of the three motorwindings W1-W3. Three voltage-sensing inputs S1-S3, in combination withan input S0 to sense the divided-down supply voltage, provide analogsensing of the voltage across each of the windings W1-W3 and across eachof the transistors T1-T3. Analog input port A/D provides currentsensing, so that abnormal motor operating conditions can be easilysensed.

When a stock condition is sensed, the motor rotation can be reversed, orintermittent current can applied in an attempt to shake the wiper bladefree; or the current can simply be cut, and an error message supplied.

The microcontroller 100 also includes an appropriate bus interface(indicated as "Control" and "Communications"), and preferably also pins(indicated as "Options") which may be connected to define user-selectedor manufacturer-selected options.

In addition, at least one additional output D4 is preferably reservedfor control of a power transistor T4 for windshield washer control.Transistor T4 controls a solenoid pump (shown as winding W4) to drivethe washer pump. (Alternatively, a brushless motor could be used for thewasher pump, and in fact this might be more economical.)

In addition, in embodiments where a single microcontroller 100 controlsmore than one wiper motor, additional output pins must be provided forcontrol of additional power transistors to drive one or more additionalmotors. (However, many microcontrollers provide dozens of latchedoutputs which can be used to control logic-level-input powertransistors.) Logic-level-input power transistors have the greatadvantage that they can be driven directly by CMOS logic levels, withoutintermediate buffer stages.

Transistors T1-T4 are preferably logic-level-input power transistors,such as an MTP50N06EL. (This specific transistor is a 50A transistor, soit is somewhat overrated for this application. For economy, smallerpower transistors would typically be used.)

FIG. 6 shows a system block diagram of an alternative system embodiment,wherein a single microcontroller directly controls multiple wiper motorsM₋₋ a and M-b. In this embodiment each of the motors is driven by itsown set of driver transistors T1aT2a/T3a or T1b/T2b/T3b. However, inalternative embodiments it is also possible to simply parallel the motorconnections to a shared set of power transistors. This is not preferred,since it does not provide the same degree of protection against a singlestuck wiper or failure of a single motor, but is an optionalalternative.

Motor Structure

FIGS. 3A and 3B show plan and isometric views of a conventional motorrotor. In this structure a number of rotor bars 310 are connected at theends by a low-resistance metal endcap/shorting bar structure 320. Themetal endcap/shorting bar structure 320 is affixed to rotor shaft 330,which rotates in bearings (not shown) to transmit torque externally.Note that, in this embodiment, the rotor bars are laterally connected bythinner metal portions 312. These thinner portions 312 not only provideadditional mechanical support to withstand the large forces on the rotorbars 310, but also contribute to the conductance of the shorting loop.(In some designs this material is made of nonferromagnetic material,such as aluminum, or of nonconductive material.)

FIGS. 3C and 3D show plan and isometric views of an innovative motorrotor modified according to the innovations disclosed herein. The rotorbars 310 are essentially the same as those in FIGS. 3A and 3B, but theendcap/shorting bar structure 320' is much thinner than endcap 320 ofthe conventional structure. (In addition, the thinner web material 312'used to laterally connect adjacent rotor bars 310 can be thinned orremoved also, to increase the effective shorting bar resistance seen bythe induced current loop; but this has not been done in the presentlypreferred embodiment.) (In the sample embodiment shown, the conventionalendcap/shorting bar structure 320 has simply been turned down to producethe thinner structure 320'.)

The prototype was demonstrated with a 1/8 hp squirrel-cage capacitor-runinduction motor with straight bars. However, the twist shown can be usedwith the polyphase motors which are contemplated as most advantageous.

FIG. 1A shows the equivalent circuit of the rotor of a conventionalvariable-reluctance motor. (Note that the structure and principles ofoperation are quite similar to those of an induction motor, except thatthe field phases are directly switched instead of being connected to thephases of an AC line.) Each of the rotor bars 310 has an intrinsicresistance, indicated here as R_(Rotor). (This resistance will typicallyhave a magnitude of less than an ohm, but this magnitude of resistancemay still be very significant in view of the large induced currentswhich typically circulate in the rotor.) The endcap/shorting barstructure is a relatively heavy metal structure, and has a resistance(as seen by the induced current loop) which is significantly smallerthan that of the rotor bars. Thus, in this circuit diagram no resistanceis indicated in the shorting bars. The induced current loop will flowthrough both endcaps, so that reducing the resistance of these endcapsreduces heat dissipation. The magnitude of the induced current willdepend on the motor's speed, horsepower, and rotor and windingdimensions, but will be much greater than the maximum drive current tothe motor, typically by a factor of the order of 10x, 30x or more.

FIG. 1B schematically shows how the resistance of the shorting barstructure is increased, as compared to the conventional structure ofFIG. 1A. The shorting bars now have a significant resistance R_(ser), asopposed to the minimal resistance of FIG. 1A. However, it is difficultto specify numerical values for these resistances, since they aredistributed resistances which must be integrated over athree-dimensional distribution which is specific to a given rotor shape.Moreover, the exact three-dimensional distribution is dependent on themagnitude of the induced current, which is dependent on the specificoperating conditions, including load. With this thickness reduction, theequivalent resistance seen by the induced current is substantiallyincreased over that of the motor of the prior art. More importantly, theequivalent resistance R_(ser) of the shorting bar structure, as seen bythe induced current loop, is now greater than the equivalent resistanceR_(rotor) of the rotor bar structure, as seen by the seen by the sameinduced current loop.

FIG. 1B schematically shows the equivalent circuit of the modified motorwith modified shorting bars, according to one class of innovativeembodiments, which provide a polyphase motor with improved capabilityfor rapid reversals, at the cost of reduced efficiency and high-speedperformance.

Further Modifications and Variations

It will be recognized by those skilled in the art that the innovativeconcepts disclosed in the present application can be applied in a widevariety of contexts. Moreover, the preferred implementation can bemodified in a tremendous variety of ways. Accordingly, it should beunderstood that the modifications and variations suggested below andabove are merely illustrative. These examples may help to show some ofthe scope of the inventive concepts, but these examples do not nearlyexhaust the full scope of variations in the disclosed novel concepts.

It should be particularly noted that the present application includesmany separate innovations, and it is not necessary to use all of themtogether. For example, the disclosed innovative control architecture canbe used in combination with a geared or flex-shaft drive instead of adirect drive.

In a further alternative embodiment, the innovative motor rotorstructure can be combined with a ferromagnetic element. The inducedcurrents in the rotor provide good running torque, and the ferromagneticelement gives some holding torque when the motor is off.

One contemplated class of alternative embodiments uses a pancake motor.Pancake motors have the advantage of high torque-to-volume ratios.Pancake motors are typically operated in 3 phase delta configuration,but can alternative be operated in a Y configuration, according to thepresently preferred embodiment.

For another example, the electrical configuration uses single-endedmotor drive. However, a double-ended driver configuration would givebetter performance, at higher cost.

For another example, the disclosed innovations can be readily adapted toother motor sizes, in accordance with the cost and torque requirementsof any particular application.

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given.

What is claimed is:
 1. A rotor for a brushless motor, comprising:aplurality of metal rotor bars which are substantially parallel along asubstantially cylindrical surface; a pair of endcaps mechanically andelectrically connected to support said rotor bars, and to electricallyconnect ends of adjacent rotor bars together; and a shaft to which saidendcaps are rigidly affixed;wherein the DC resistance between twoadjacent ones of said rotor bars, through one of said endcaps, is equalto or greater than one-half of the DC resistance along the length of oneof said rotor bars.
 2. The rotor of claim 1, further comprising a thinmetallic webbing physically connecting adjacent ones of said rotor bars,between said endcaps.
 3. The rotor of claim 1, further comprising a thinmetallic webbing physically connecting adjacent ones of said rotor bars,between said endcaps, and wherein said webbing is not ferrous.
 4. Therotor of claim 1, wherein said rotor bars each have a helical surfacewhich lies within a cylinder centered on said shaft.
 5. A rotor for abrushless motor, comprising:a plurality of metal rotor bars which aresubstantially parallel along a substantially cylindrical surface; a pairof endcaps mechanically and electrically connected to support said rotorbars, and to electrically connect ends of adjacent rotor bars together;metallic webbing, which is thinner than said rotor bars, laterallyconnecting adjacent ones of said rotor bars; and a shaft to which saidendcaps are rigidly affixed;wherein a DC resistance between two adjacentones of said rotor bars is equal to or greater than one-quarter of theDC resistance along the length of one of said rotor bars in isolationfrom said webbing and endcaps.
 6. The rotor of claim 5, wherein saidwebbing comprises aluminum.
 7. The rotor of claim 5, wherein said rotorbars each have a helical surface which lies within a cylinder centeredon said shaft.
 8. A rotor for a brushless motor, comprising:a pluralityof metal rotor bars which are substantially parallel along asubstantially cylindrical surface; a pair of endcaps mechanically andelectrically connected to support said rotor bars, and to electricallyconnect ends of adjacent rotor bars together; and a shaft to which saidendcaps are rigidly affixed;wherein, when said shaft, endcaps and rotorbars are rotatably mounted in a polyphase rotating magnetic field statorstructure, multiple current loops will be induced in said rotor bars andendcaps such that a resistance seen by the path of each said currentloop through one of said endcaps, is equal to or greater than one-halfof the resistance seen by the portion of said current loop which flowsalong the length of one of said rotor bars.
 9. The rotor of claim 8,further comprising a thin metallic webbing physically connectingadjacent ones of said rotor bars, between said endcaps.
 10. The rotor ofclaim 8, further comprising a thin metallic webbing physicallyconnecting adjacent ones of said rotor bars, between said endcaps, andwherein said webbing is not ferrous.
 11. The rotor of claim 8, whereinsaid rotor bars each have a helical surface which lies within a cylindercentered on said shaft.
 12. A brushless motor, comprising:a plurality offixed windings having multiple windings and external connections to saidwindings; a shaft rotatably mounted in bearings to rotate within saidwindings; a pair of endcaps rigidly affixed to said shaft; a pluralityof metal rotor bars which are mechanically and electrically connected tobe supported by said endcaps, said endcaps electrically connecting endsof adjacent rotor bars together; said shaft, endcaps, rotor bars andbearings having a spatial relation such that, when said shaft rotates,said rotor bars are closely inductively coupled to magnetic fieldsgenerated by said windings, such that multiple current loops will beinduced in said rotor bars and endcaps;wherein a resistance seen by thepath of each said current loop through one of said endcaps, is one-halfof the resistance seen by the portion of said current loop which flowsalong the length of one of said rotor bars.
 13. The motor of claim 12,comprising only three said external connections to said windings. 14.The motor of claim 12, further comprising a thin metallic webbingphysically connecting adjacent ones of said rotor bars, between saidendcaps.
 15. The motor of claim 12, further comprising a thin metallicwebbing physically connecting adjacent ones of said rotor bars, betweensaid endcaps, and wherein said webbing is not ferrous.
 16. The motor ofclaim 12, wherein said rotor bars each have a helical surface which lieswithin a cylinder centered on said shaft.
 17. An electrical system,comprising:a polyphase brushless motor connected without gearing to turna mechanical load element; and control logic connected to reverse saidmotor frequently;wherein said motor comprises a rotor having a shaftrotatably mounted in bearings to rotate within said windings; a pair ofendcaps rigidly affixed to said shaft; and a plurality of metal rotorbars which are mechanically and electrically connected to be supportedby said endcaps, said endcaps electrically connecting ends of adjacentrotor bars together; said shaft, endcaps, rotor bars and bearings havinga spatial relation such that, when said shaft rotates, said rotor barsare closely inductively coupled to magnetic fields generated by saidwindings, such that multiple current loops will be induced in said rotorbars and endcaps; wherein a resistance seen by the path of each saidcurrent loop through one of said endcaps, is equal to or greater thanone-half of the resistance seen by the portion of said current loopwhich flows along the length of one of said rotor bars.
 18. The systemof claim 17, further comprising a thin metallic webbing physicallyconnecting adjacent ones of said rotor bars, between said endcaps. 19.The system of claim 17, further comprising a thin metallic webbingphysically connecting adjacent ones of said rotor bars, between saidendcaps, and wherein said webbing is not ferrous.
 20. The system ofclaim 17, wherein said rotor bars each have a helical surface which lieswithin a cylinder centered on said shaft.